Rolling bearing are mounted to movable portions of power transmission mechanisms or travel units of automobiles and various kinds of construction machines, or to movable portions of motors, hydraulic pumps, or various kinds of industrial machines. If such a rolling bearing is used under the circumstances that lubrication is not sufficient due to low film thickness ratio A, etc. or used under relatively harsh conditions, wear powder (such as iron powder) or another type of foreign substance might go into the rolling bearing.
If due to the continuous operation of a machine, a large amount of foreign matter goes into the lubricant oil with a bearing lubricated insufficiently, a portion of the bearing might be damaged, which might shorten the service life of the bearing.
In order to extend the service life of a bearing, it is known to mount to the bearing a filter for preventing foreign matter from going into the bearing space of the bearing. Specifically, it is know that a seal ring closing one side of the bearing space of a bearing is formed with an oil passage hole through which oil flows, and a filter for catching foreign matter is fitted in the oil passage hole (e.g., see the below-discussed four Patent documents).
Japanese Unexamined Patent Application Publication No. 1106-323335 discloses a rolling bearing including a filter of which the porosity is determined on the basis of how the bearing is used, the required optimum service life of the bearing, or lubrication performance in the bearing (see paragraph [0009] of that reference). However, the rolling bearing might not be able to perform two functions under harsh use conditions. Specifically, the bearing might not enable the smooth flow of lubricant oil into and out of the bearing space, and might not prevent the entry of foreign matter into the bearing space. If the mesh portion of the filter is set to be small such that the filter can reliably catch foreign matter, the filter might be clogged with foreign matter, thereby making it difficult to lubricate the bearing sufficiently.
Japanese Unexamined Patent Application Publication No. 2002-250354 discloses a bearing including a filter assembly comprising an internal filter having a fine mesh portion, and an external filter having a coarse mesh portion. Since the mesh portion of the external filter is coarse, and the mesh portion of the internal filter is fine, it is possible to prevent the filter assembly from being clogged with foreign matter, and to remove foreign matter. However, according to confirmation tests, even the use of a filter having a mesh size smaller than the size of foreign matter tends to shorten the service life of the bearing (see comparative examples 2, 3, 4 of that reference). It is estimated from these tests that due to the fact that the mesh size of the filter is not uniform, and the size of foreign matter is also not uniform, the mesh portion of the filter is partially larger than some foreign matter (“mesh size>foreign matter size”), so that some foreign matter has passed through the mesh portion of the filter.
In such a filter structure, foreign matter caught by a filter might be stirred by the flow of lubricant oil, and float in the lubricant oil again.
In order to overcome such a problem, Japanese Unexamined Patent Application Publication No. 2012-202417 discloses a bearing in which a permanent magnet is mounted in the vicinity of the mesh portion of a filter such that foreign matter is attracted to the magnet due to the magnetic force thereof. However, with this arrangement, the permanent magnet tends to attract thereto foreign matter existing outside of the bearing, too. Moreover, the permanent magnet is directly exposed to the outside of the bearing space of the bearing, so as to attract more foreign matter thereto. Therefore, if due to the continuous operation of the bearing, the amount of foreign matter attracted to the permanent magnet exceeds the magnetic force of the permanent magnet (such that the permanent magnet is unable to attract thereto the foreign matter by its magnetic force), a large amount of foreign matter attracted to the permanent magnet might separate from the permanent magnet simultaneously all together.
Japanese Unexamined Patent Application Publication No. 2012-102767 discloses a bearing in which captured foreign matter does not fall into the bearing. However, this structure makes a filter and a seal ring large. Also, this structure is effective for foreign matter going into the bearing from the outside of the bearing, but is not effective for small foreign matter able to pass through a filter, or for wear powder of bearing parts generated in the bearing.
It is an object of the present invention to more reliably prevent foreign matter from floating in the bearing space of a rolling bearing, while maintaining lubrication performance in the rolling bearing.